Disk files are information storage devices which utilize a rotatable disk with concentric data tracks containing the information, a head for reading or writing data onto the various tracks, and an actuator connected by a support arm assembly to the head for moving the head to the desired track and maintaining it over the track centerline during read or write operations. The movement of the head to a desired track is referred to as track accessing or "seeking", while the maintaining of the head over the centerline of the desired track during a read or write operation is referred to as track "following".
The actuator is typically a "voice coil motor" (VCM) which comprises a coil movable through the magnetic field of a permanent magnetic stator. The application of current to the VCM causes the coil, and thus the attached head, to move radially. The acceleration of the coil is proportional to the applied current, so that ideally there is no current to the coil if the head is perfectly stationary over a desired track.
In disk files which have a relatively high density of data tracks on the disk, it is necessary to incorporate a servo control system to maintain the head precisely over the centerline of the desired track during read or write operations. This is accomplished by utilizing prerecorded servo information either on a dedicated servo disk or on sectors angularly spaced and interspersed among the data on a data disk. The servo information sensed by the read/write head (or the dedicated servo head if a dedicated servo disk is used) is demodulated to generate a position error signal (PES) which is an indication of the position error of the head away from the nearest track centerline. One type of conventional servo pattern for use with either a sector servo disk or a dedicated servo disk is the quadrature pattern described by Mueller, et al. in IBM Technical Disclosure Bulletin, Vol. 21, No. 2 (Feb. 1978) pp. 804-805. In the quadrature pattern there are four unique track types which repeat so as to form radially repetitive four-track bands of servo information.
During track seeking, when the head is moving across the tracks, the PES is used to generate track crossing pulses. This track crossing information, together with the PES and a signal representing the desired or target track, is used to generate a total error signal. The total error signal is equal to the sum of the PES plus the difference between the position of the target track and the position of the actual track over which the head is located. The total error signal is then used in the servo feedback loop to compute the desired velocity of the head, via a reference velocity trajectory generator, to assure that the head arrives at the target track according to the optimum velocity trajectory to effect a move to the target track in minimum time. The computed velocity is then compared with the estimated velocity from an electronic tachometer to generate a velocity error signal to a power amplifier which in turn applies a control current to the VCM. An electronic tachometer for generating a velocity estimate from inputs of VCM control current and quadrature PES is described in U.S. Pat. No. 4,246,536 to Bradley, et al., which is assigned to the same assignee as this application. In some disk files the estimated velocity is provided by a mechanical tachometer, such as a linear variable differential transformer (LVDT).
During track following, when the head is located within the boundaries of the desired track, the PES alone is used in the servo feedback loop to generate a control signal to the VCM to move the head back to the track centerline.
A description of operation of a general disk file servo control system during track seeking and track following is given by R. K. Oswald in "Design of a Disk File Head-Positioning Servo", IBM Journal of Research and Development, November 1974, pp. 506-512.
In such conventional disk files, the use of track crossing pulses to determine the total error signal during a track seek requires additional costly and complex analog circuitry in the demodulator to generate the track crossing pulses from the PES. More importantly, in the case of disk files which use sector servo data, it is not possible to accurately count track crossing pulses directly from the PES since the head will typically have crossed numerous tracks between PES samples.
During operation of the disk file the VCM is subjected to certain generally constant or very low frequency forces which tend to bias the coil, and thus the attached head, away from the desired position, i.e. the position needed to follow the desired velocity trajectory during track seeking, or the position of the centerline of the desired track during track following. These bias forces are caused by such things as circulating air flow passing the coil and the arm assembly which connects the head to the coil, the movement of the flexible ribbon conductor connecting the coil and arm assembly to the read/write electronics of the disk file, friction between the bearings and guide rails of the actuator, and the gravitational effect on the coil and arm assembly if the radial access direction of the VCM is not perfectly aligned horizontally. In addition to bias forces, there is typically an input voltage offset in the power amplifier which supplies the control current to the VCM, so that even in the absence of a velocity error signal to the power amplifier, the VCM may experience a non-zero input current.
The bias forces and voltage offset have little effect on the performance of the servo control system during track seeking when the primary contribution to the total force on the actuator is that due to the current in the actuator. However, at the completion of a track seek and during track following, when the head is within approximately one-half track width of the centerline of the target track and when the total error signal is simply the PES, the bias forces and voltage offset are primary contributions to the PES and must be rapidly and effectively compensated by the control signal to the power amplifier to produce current in the VCM to maintain the head over the track centerline.
One technique for rapidly forcing the total error signal to zero during track following is to incorporate an integrator for the PES into the servo feedback loop. This is accomplished by the use of a separate "stiffness" integrator which is switched into the servo feedback loop only during track following. However, use of the switchable stiffness integrator generates an electrical transient which occurs as a result of switching of the integrator when large system bias forces and voltage offset is present. In addition, the stiffness integrator must be switched in at precisely the proper moment. Both the transient caused by switching of the stiffness integrator and switching at the improper time can cause overshoot or undershoot of the head with respect to the centerline of the target track which thus delays the subsequent read or write operation.
An additional problem caused by bias forces in all disk files is that there is a gain in the feedback path around the power amplifier in the presence of a generally constant or very low frequency PES. In the case of disk files which use an electronic tachometer, this is because the electronic tachometer receives as input the VCM control current and the measured PES and, through a combination of control current integration and PES differentiation, estimates the velocity of the head. Thus, even if the head is stationary, but the coil and arm assembly is subjected to generally constant or very low frequency bias forces, the electronic tachometer will output a non-zero velocity estimate. In the case of disk files which use a mechanical tachometer, electrical offsets cause a non-zero output of estimated velocity even when the head is stationary.